1. Field of the Invention
This invention relates to the preparation of polyamines and is more particularly concerned with the continuous preparation of methylene-bridged polyphenyl polyamines by acid condensation of aniline and formaldehyde.
2. Description of the Prior Art
The acid condensation of aniline and formaldehyde to produce a mixture of methylene-bridged polyphenyl polyamines containing a major portion of di(aminophenyl)methane is well-recognized in the art; see, for example, U.S. Pat. Nos. 2,938,054; 3,163,666; 3,260,751; 3,274,247; and 3,277,173. The mixture of polyamines can be used for a variety of purposes, for example, as a source of pure di(4-aminophenyl)methane (MDA) which is useful as an epoxy resin curative and an intermediate in the preparation of di(4-aminocyclohexyl)methane (H.sub.12 MDA). Both the MDA and H.sub.12 MDA are useful in the preparation of polyamides; see, for example, U.S. Pat. Nos. 2,669,556 and 3,416,302, respectively.
Alterntively, the mixture of polyamines can be phosgenated, by procedures well-known in the art, to the corresponding mixture of polymethylene polyphenyl polyisocyanates containing methylenebix(phenyl isocyanate) as the major component. The latter can be recovered from said mixture, if desired, and finds broad application in the preparation of elastomeric and other non-cellular polyurethanes. In addition, the mixture of polymethylene polyphenyl polyisocyanates obtained in the above phosgenation is widely used in industry in the manufacture of cellular polyurethanes. Recently there has been described a highly useful process of recovering methylenebis(phenyl ioscyanate) and a mixture of polymethylene polyphenyl polyisocyanates continuously and simultaneously from a polymethylene polyphenyl polyisocyanates feedstock containing a high proportion (65 to 75 percent by weight) of methylenebis(phenyl isocyanates); see, British Patent No. 1,092,019.
For many of the purposes for which the di(aminophenyl)methane and methylenebis(phenyl isocyanates), obtained as described above, are to be used, it is desirable that these materials be prepared in the form of the substantially pure 4,4'-isomer (i.e. at least 98 percent by weight). The presence of significant amounts of the corresponding 2,4'- and 2,2'-isomers in these materials is undesirable particularly where the materials are to be used in the preparation of polyamide and polyurethane fibers and filaments.
Hitherto it has been necessary to submit the diamine or diisocyanate obtained as described above to purification procedures such as fractional distillation, fractional crystallization, and the like, in order to obtain the desired 4,4'-isomers free from significant amounts of the corresponding 2,4'- and 2,2'-isomers. Such purification methods are cumbersome on the manufacturing scale and add significantly to the cost of producing the end-product.
We have now found that it is possible, by means of the novel process described hereinafter, to produce directly, in a continuous manner, a mixture of polyamines in which the di(aminophenyl)methane component is present in the form of the substantially pure 4,4'-isomer (purity of at least 98 percent by weight) and thereby to avoid the necessity to embark upon extensive purification of this diamine, or of the corresponding diisocyanate to which it is converted by phosgenation. This finding is of considerable commercial importance and enables the production of the afore-mentioned diamine and diisocyanate to be accomplished at considerable savings in cost.
Further, the process of the present invention can be used to prepare methylene-bridged polyphenyl polyamines which are phosgenated to produce the feedstock for the process described in the aforesaid British Patent No. 1,092,019. It is possible thereby to follow a remarkably economic route to the simmultaneous production of substantially pure 4,4'-methylenebis(phenyl ioscyanate) and polymethylene polyphenyl polyisocyanates.
These various advantages in the process of the present invention mark a clear advance over previously described continuous processes for the production of methylene-bridged polyphenyl polyamines.